188 



MAMMALIAN ANATOMY 



fossae. It contains three cavities, whereof two are in the auditory 

 bulla and one is in the substance of the bone itself and contains the 

 complicated organ of hearing, and hence is known as the cavity of 

 the internal ear. One of the cavities in the bulla is the tympanum, 

 or cavity of the middle ear. 



The temporal consists of four parts, which correspond, in a meas- 

 ure, with the parts from which it is developed. 



First, above is the thin, shell-shaped, squanious portion or squa- 

 mosal, with its zygomatic process (Fig. 126) ; second, behind is the 



FIG. 126. 



OUTER VIEW. INNER VIEW. 



PARTS OF THE TEMPORAL BONE. 



swollen mastoid ; third, on the inside is a white, hard, pyramidal 

 mass, the petrous portion or petrosal. The under surface of the 

 petrous would be visible on the base of the skull were it not for 

 the fourth part, the tympanic, forming the auditory bulla. The 

 bulla consists of two parts, separated by a groove parallel with its 

 long axis arid very near the lower margin of the auditory meatus. 

 The outer part is denser, much smaller than the inner, and is 

 developed from a separate ring of bone, the tympanic bone. The 

 bulla can be removed from the rest of the temporal, at its points 

 of attachment behind, and in front of the auditory meatus ; there 



*.* 



are then exposed not only the inferior surface of the petrous, but also 

 parts of the squanious and mastoid, which, with the petrous and part 

 of the sphenoid bone, form the roof of the tympanum, whereof the 

 floor is contributed by the bulla itself (Fig. 127). For convenience 

 the bulla may be removed and described after the consideration of 

 the three other parts. The tympanum will be studied separately. 



The squanious and petrous portions coalesce behind with the 

 mastoid, but for the rest of their circumference are free. 



